Medical radiography is a well-known and extremely valuable tool for the early detection and diagnosis of various disease states of the human body. However, body cavities and the soft tissue of body organs and blood vessels absorb so little X-ray radiation that radiographs of these body portions are difficult to obtain. To overcome this problem, radiologists introduce radiographic contrast agents, sometimes referred to as radiopaques, into such body cavities and tissues. Such contrast agents have been used in the X-ray examination of the human body almost from the days of Roentgen.
The book Radiographic Contrast Agents, by R. E. Miller and J. Skucas, University Park Press, 1977, at page 3-5, discloses a variety of materials which have been proposed as contrast agents. These materials include, for example, potassium halides, bismuth subnitrate, bismuth subcarbonate, heavy metals such as cerium, thorium, mercuric sulfide, iron, and zirconium compounds; certain heavy metal chelates; barium titanate; various iodinated organic compounds; and barium sulfate, the latter being the most widely used contrast agent for the gastrointestinal tract.
U.S. Pat. No. 3,832,457 discloses yet another type of radiographic contrast composition which contains finely-divided solid particles of at least one kind of soft magnetic ferrite dispersed in a liquid carrier together with an organic thickening compound, such as starch, and a fine powder of a metallic oxide such as a rare earth oxide, e.g., lanthanum oxide.
Although many different contrast agents have been used in the X-ray examination of the gastrointestinal tract, U.S. Pat. No. 3,368,944 issued Feb. 13, 1968 discloses that the agents thus far employed are far from perfect and often cause undesired effects. Toxicity, of course, is a primary concern because many of the elemental metals present in prior art contrast agents are clearly highly toxic materials. For example, the barium ion is a highly toxic material. Thus, a barium sulfate contrast agent, must be highly purified to eliminate any soluble barium ions.
Other undesired effects also exist. For example, the constipating tendency of barium sulfate is well-known among radiologists, and even small amounts of intravenous barium sulfate act as a purgative. See Radiographic Contrast Agents, supra, at page 135. In addition, barium sulfate is well-known as a strong adsorber of all sorts of materials, including a variety of cations, anions, various additives in which the barium sulfate may have been processed, and the like. Because of these strong adsorption properties, barium sulfate contrast agents are easily contaminated. Thus, radiologists must be extremely careful in the selection and use of barium sulfate-containing contrast agents.
Based on the foregoing, the need for the development and use of new contrast agents is readily apparent.